This episode is narrated in five "acts", with title cards displaying the start of each act.
Act I - Tony Warner is playing the titular role in Macbeth on London's West End. Backstage in his dressing room after a performance, Tony complains about his career taking a downturn and yells at dresser Kirstie. It is mentioned that he is a recovering alcoholic but has been "dry" for several years. His understudy Jim invites him to attend the understudy rehearsal, but Tony lies about having another job at that time. His neighbours arrive, having come specially to watch him; he is unable to wriggle out of being invited to dinner with them. Jim tries on Tony's prop crown but hears a roll of thunder and sees Tony's face in front of him.
Act II - Jim and his fiancée Laura, the understudy for Lady Macbeth, are in Tony's dressing room preparing for the understudy rehearsal. Laura puts pressure on Jim to push for Tony's job. Felicity, the company manager, makes an excuse for Jim to leave so that she can hit on Laura. Laura opens Tony's payslip to find out how much he earns; upon seeing the figure, she encourages Jim to find a way to replace Tony. She cuts herself on a pin left in her costume, and yells at Kirstie for leaving it there. Laura repeats Lady Macbeth's Act I, Scene V "Make thick my blood ..." speech, reiterating that she's serious about wanting Jim to replace Tony. Just as they're about to go onstage Jim is distracted by a drop of blood on the floor; which grows into a spreading pool. Laura calls his name and Jim snaps out of it, realizing there is no blood.
Act III - During a performance, Kirstie tries to help a drunken Tony get ready for Act V of the play. Tony denies being drunk and says he's only been drinking the smoothies Kirstie makes for him. Felicity puts Tony in the shower and tells Jim he'll have to go onstage if Tony doesn't sober up quickly. Laura comes in and pressures Jim to take Tony's place, but he's not sure he can do it - he would have to perform Macbeth's big fight scene with Macduff, which the understudies have not rehearsed properly. When Laura tries to hand him a prop dagger, he sees blood all over the blade and her hands. Tony reprises his role and Laura yells at Jim for missing his chance and lacking the ambition to achieve his dream. She angrily takes off her engagement ring, but then hears noises from inside the shower; when she looks, it is empty. There is suddenly the sound of Tony screaming, and someone calls over the speakers for a medic. Laura puts the ring back on and smiles.
Act IV - Jim prepares to take over the role of Macbeth for the first time; Tony is in hospital with spinal injuries. Laura asks after Tony and tries to make conversation, but Jim is distant towards her. Felicity was recently fired for sexual harassment, and Jim says he doesn't think her behaviour went that far. He seems to suspect Laura of reporting her, although Laura denies it. Jim declines Laura's invitation to go out to dinner, and eventually asks her to leave, feeling that he needs to concentrate on his performance. He thanks her for "this" and Laura nods. Jim puts on his crown and prepares to go onstage. He hears whispering over the speaker system and sees a horrific vision of blood pouring from his eyes and the mirror; which stops when he takes the crown off again.
Act V - Nineteen months later, Jim is a famous actor and has returned to the same theatre to play the lead in Richard III. Tony, now using a wheelchair, stops by to visit. He asks after Laura, and Jim explains they broke up and haven't seen each other in forever. Tony leaves and Kirstie, who now takes care of him full time, comes to see Jim. She says that she watches every one of Jim's performances and always knew he would be a star - he just needed "a little push." She mentions the dressing room was redecorated because Laura killed herself in the shower; upsetting Jim, who had no idea Laura was dead. He says he believes Laura was responsible for Tony's accident and couldn't live with the guilt. Kirstie says she herself did everything: giving Tony alcohol, pushing him, and getting Felicity fired because she wouldn't let Jim perform. Kirstie shows Jim that she's wearing Laura's engagement ring, and says "I'm waiting in the wings, like an understudy. Have a good show ... I'll be watching." She leaves, and Jim sees blood everywhere and visions of Laura's death.
This episode contains examples of:
- Ambiguous Ending: Did Laura kill herself, and Kirstie take the engagement ring from her corpse? Or did Kirstie kill Laura and loot her body? The latter is more likely, but both situations are possible.
- Blatant Lies: Tony claiming to have another job at the exact time of the understudy rehearsal.
- Casting Couch: Tony says that one of the other cast members gets roles this way.
- Driven to Suicide: Apparently Laura, although it's much more likely Kirstie killed her.
- Framing the Guilty Party: Felicity is fired after being accused of sexual harassment. Although the accusation later turns out to have been made up by Kirstie, Felicity is in fact guilty of sexual harassment, as she was shown harassing Laura earlier in the episode.
- Groin Attack: At one point, the cast has to rehearse a fight scene without the fight director, who is in hospital because "Derek Jacobi caught him in the goolies with a nunchuck"
- Lady Macbeth: Both Laura, who is this on and off the stage, and Kirstie.
- Loony Fan: Kirstie is prepared to do awful things for Jim.
- Love Makes You Evil: Kirstie. Also Laura, who was at least clearly prepared to do something bad for Jim to take Tony's place.
- Maybe Magic, Maybe Mundane: Jim foresees violent events, in particular Laura's suicide, but the cause is never specified either way. They could just be hallucinations inspired by the violent script and his guilt.
- Only in It for the Money: Tony still has to do advertising voiceovers for money despite being a leading theatrical actor.
- Only Sane Man: Felicity is comparitively this for the whole cast and crew. She makes unwelcome attempts at flirting with Laura, and she is extremely rude to the understudies. But she is the only member of the production who does not do something awful or go along with such an act.
- Psycho Lesbian: Felicity has some elements of this. Her flirting makes Laura (who's already engaged to Jim) uncomfortable, and she's later fired for sexual harassment. Ultimately subverted in that it turns out she was innocent - Kirstie set her up because Felicity wouldn't allow Jim to go onstage as Macbeth.
- Shout-Out: To Doc Martin, Game of Thrones, and the actress Michele Dotrice, among others.
- Slipping a Mickey: Kirstie did this before Tony's accident.
- Tears of Blood: During one of Jim's hallucinations.
- Whole-Plot Reference: To Macbeth itself.
